The U.S. Pat. No. 4,935,791 descries a nonvolatile memory device storing a couple of bits of data therein. The U.S. Pat. Nos. 5,768,192; 6,011,725; 6,255,166 and 6,399,441 also describe a nonvolatile memory device in which plural bits of data can be stored in a single memory cell.
According to a conventional type of nonvolatile memory, the memory is unstable in operation due to unevenness of parasitic resistance.